1. Field of the Invention
The present invention relates to a fixing controller and an image forming apparatus including the same.
2. Description of the Related Art
An image forming apparatus generally includes a fixing unit for thermally melting toner of toner image, which is transferred but unfixed to a sheet, and for fixing the toner image onto the sheet.
There are known fixing units configured to rapidly raise the toner temperature, e.g., by having a thin-walled small diameter fixing roller (heating medium) or by having a heating member disposed inside and in contact with a rotor. In recent years, a fixing unit of an electromagnetic induction heating type has also been used that has a thin-walled metallic rotor for generating heat by induction heating. Each of these fixing units includes a rotor (heating medium or heat-generating medium) configured to have a small thermal capacity so as to be efficiently heated by a heat source.
For image forming apparatuses, e.g., copying machines, there have been proposed various fixing units of a type having a thin-walled rotor made in contact with a sheet so as to heat and melt a toner on the sheet.
However, in a case that the thin-walled rotor, which is small in thermal capacity, is used as a heating medium and heat is locally generated by, e.g., electromagnetic induction heating, it is difficult to achieve satisfactory heat transfer from a heat generating part to other parts. Such a tendency becomes more noticeable when the rotor is made thinner to shorten a warm-up period.
The above unsatisfactory heat transfer does not cause a problem in a case that the rotor rotates during heating. On the other hand, if the rotor stops rotating during the heating, this is equivalent to the heat generating part becoming further smaller in thermal capacity, which results in a rapid temperature rise.
In a case that the rotor stops rotating during heating, the speed of detection by temperature detecting means lags behind the speed of temperature rise. As a result, when an abnormal temperature rise takes place, there occurs a delay in the timing of excessive temperature rise detection for stopping the heating, which poses a problem that the fixing unit can be damaged.
For example, such a delay in excessive temperature rise detection causes a fear that peripheral members made of resin are shortened in heat life and thermally damaged.
In this regard, Japanese Laid-open Patent Publication No. 2005-338698 proposes a fixing unit of local heating type, which is configured to have a small heat capacity by using a thin-walled roller or belt. The proposed fixing unit has a heating member, held by a film support disposed in a cylindrical film, for heating a sheet passing between the cylindrical film and a pressure roller which is in pressure contact with the cylindrical film. This fixing unit controls the temperature of the heating member to a predetermined temperature based on the heating member temperature detected by a thermistor (temperature detecting means), and determines the possibility of occurrence of a slip between the film and the sheet on the basis of temperature rise speed.
Japanese Laid-open Patent Publication No. 2005-24934 proposes a technique for detecting a speed detection pattern formed on a belt by an optical rotation detecting means to detect a belt drive speed, and determining whether the belt rotates or stops rotating based on the detected speed.
The fixing units proposed in Japanese Laid-open Patent Publication Nos. 2005-338698 and 2005-338698 are each configured to stop the heating operation when it is determined that a slip takes place between film and sheet or when the belt does not rotate normally due to, e.g., abnormality of a driving motor.
However, with the slip determination based on the temperature rise speed, an occurrence of slip can be determined if the slip takes place during temperature rise from a low temperature to a target temperature, but cannot accurately be determined if the slip takes place after the film is already heated up to some extent.
With the technique of detecting the belt rotation by the rotation detecting means, it is necessary to newly add the rotation detecting means, resulting in increase in cost and size. In addition, the rotation detecting means for use in high temperature environment is difficult to install.